Among the widely-used instruments for measuring surface topographies are interferometers, which use the wave nature of light to map variations in surface height with a high degree of accuracy. However, when a transparent flat plate with parallel surfaces is illuminated with a laser-based interferometer, the resulting interference pattern is a complex mixture of fringes created by reflections from both the front and back surfaces of the plate. In order to do meaningful metrology on such an object, the common practice is to either thinly coat the front surface with a high reflectivity material, or to defeat the back-surface reflection by applying some kind of index-matching coating. It would be desirable to dispense with these kinds of surface treatments for regular inspection and testing, particularly for process control in a production environment.
Measurement problems relating to multiple or spurious reflections in interferometers are well known. In an article entitled Digital Wave-Front Measuring Interferometry. Some Systematic Error Sources, by J. Schwider, et al., 22 Applied Optics 3421 (1983), the deleterious effects of spurious reflections are described in some detail. More recently, an article entitled Testing An Optical Window Of A Small Wedge. The Effect Of Multiple Reflections, by C. Ai and J. C. Wyant, 32 Applied Optics 4904 (1993), discussed the problem of compound reflections between the front and back sides of a transparent flat object during a transmitted-wavefront test. However, these articles and many other articles and patents related to optical metrology do not teach specific apparatus and means for measuring the surface height variations of transparent objects having approximately parallel flat surfaces. Substantially the same difficulty arises when working with transparent objects having approximately concentric spherical surfaces, such as domes and contact lenses.
The generally accepted prior art principle is that transparent objects that produce multiple reflections in interferometers can only be accurately measured if one or the other of the reflections is greatly reduced or eliminated by treating the surface in question, or by employing a more complex optical geometry. For example, several prior-art techniques that reduce the effect of multiple reflections employ spatially incoherent sources or extreme angles of illumination, such as is described in U.S. Pat. No. 4,325,637, entitled Phase Modulation Of Grazing Incidence Interferometer, to R. C. Moore. These techniques, however, generally sacrifice measurement accuracy, optical simplicity, ease of use and cost.
There is accordingly an unmet need for an accurate and convenient method and apparatus for obtaining surface profiles of transparent objects.